New Delhi: Prime Minister Narendra Modi Monday expressed deep sadness over the loss of lives in an explosion in a metro train in Saint Petersburg.

Prime Minister Narendra Modi. Reuters

"Deeply saddened by the loss of lives in the blasts at Saint Petersburg metro. Heartfelt condolences to the families of the victims," he tweeted.

At least 10 people were killed and 50 were injured on Monday in an explosion in Saint Petersburg.

President Vladimir Putin, who was visiting the city on an unrelated trip, said investigators were looking into whether the explosion was a terror attack or if there might have been some other cause.

Russia's National Anti-Terrorist Committee said an unidentified explosive device went off on a train that was travelling between two stations. A second bomb was found at another station in Saint Petersburg, but it was diffused on time, reports said.